In the oil and gas exploration and production industry, it is common to cut, mill, grind or drill through steel components such as casing in an installed wellbore, for example to form a window in the wellbore to allow a sidetrack well to be drilled. The material removed by this process (referred to as swarf) is mixed with the drilling fluid (or mud), which is circulated through the wellbore and returned to surface via the wellbore annulus along with the drill cuttings. It is desirable to process the drilling mud returns to remove the drill cuttings for treatment and disposal, and to prepare the drilling mud for recirculation. The swarf is highly erosive and must be removed from the valuable drilling mud to allow it to be reused safely. However, significant quantities of swarf in drilling mud returns may interfere with or damage surface flow equipment including equipment used for the separation of solid particles (such as drill cuttings or rock fragments), presenting the operator with an additional problem.
The ferrous nature of swarf has led to proposals to use magnetic fields to separate the swarf from the fluid. U.S. Pat. No. 3,476,232 describes an apparatus for batch treatment of drilling fluid, which includes a series of magnetic bars on a conveyor inside a casing. The magnetic bars lift the swarf from a vessel and cause it to be dropped in a collection chamber. The U.S. Pat. No. 3,476,232 apparatus has a geometry which provides only low magnetic field penetration into the liquid. It is slow in operation and is limited in its application to the treatment of a flowing liquid. U.S. Pat. No. 3,476,232 does not provide a means for separating non-magnetic solid particles from the liquid to be treated.
US 2005/0045547 describes a magnetic separator apparatus which has a pair of conveyor chains from which are suspended frames with spaced-apart magnetic rods. The magnetic rods hang vertically in the liquid as the chains follow their path through the liquid, and the liquid in the tank flows through the frames. The rods are cleaned at a wiping station.
DE 4337484 and EP 0532136 describe magnet separator systems which include a series of circulating magnetic rods driven by chains through a liquid. At a collection location, wipers for the magnetic rods are activated to remove accumulated magnetic particles.
U.S. Pat. No. 6,355,176 describes an assembly and method for collecting and releasing magnetic materials which includes elongated permanent magnets arranged to cyclically move through a tank of liquid and be moved to a collection location at which the magnets are separated from their covers.
WO 07/23276, filed by the present applicant, describes an improved apparatus which uses a series of pipes which contain circulating magnetic chains. The pipes pass through a channel through which swarf-containing drilling mud flows from a drilling operation. The magnetic chains attract the particles to the outside of the pipe, and transport them along the pipe until they are released into a collection chamber. The apparatus may be used in conjunction with an array of elongate magnets located in housings which are supported in a partially submerged position in the flow channel. Swarf particles are attracted to the outside of the housings, which may be removed from the flow channel. Displacement of the magnet with respect to the housing releases the swarf particles into a collection chamber.
The arrangement of WO 07/23276 improves upon U.S. Pat. No. 3,476,232 and other previously proposed systems by virtue of its geometry, reliability and configurability, and has been successfully used in commercial applications. However, it does not provide a mechanism for the separation of non-magnetic solid particles from a drilling fluid. In addition, it is generally desirable to increase the exposure of the flowing liquid to a magnetic field; increase the flow rate of fluid that may pass through the apparatus; and reduce the size or footprint of the apparatus for offshore use.
It is therefore an aim of the present invention to provide an apparatus for handling oil and gas process liquids or slurries and a method of use which addresses one or more drawbacks or deficiencies of the previously proposed apparatus and methods.
One aim of the invention is to provide an improved apparatus for and method of removing magnetic swarf particles from an oil or gas process liquid (such as drilling mud). An additional aim is to provide an apparatus for and method of separating non-magnetic and magnetic swarf particles from a liquid flowing from an oil or gas operation (such as drilling mud).
Additional aims and objects of the invention will become apparent from reading the following description.